The invention relates to a strip guiding device of the type having a rotatable support construction with supporting rolls.
Strip guiding devices of this nature are known, inter alia from DE-C-3401792. This document describes a strip guiding device which is intended to guide hot metal strip over supporting rolls between or inside horizontal furnace chambers of a continuous annealing furnace. In this case, the metal strip hangs freely between two or more strip guiding devices as it moves through the furnace chambers. Continuous annealing furnaces are in widespread use, for example, for the oxidative annealing of stainless strip in a so-called annealing and pickling line. These furnaces operate at high temperatures and high production capacities. The strip guiding device in DE-C-34 01 792 comprises a support construction in roll form which is rotatably accommodated in a frame. Two diametrically opposite, cylindrical cavities are formed in the roll-like support construction, in which cavities two supporting rolls are accommodated in a freely rotatable manner. The roll-like support construction substantially comprises a solid body with cooling lines which extend inside it. The solid body provides insulation against the high temperatures in the furnace chamber for the supporting roll which is not in operation. Maintenance on the supporting roll which is at the top in operation can be carried out by rotating the support construction through 180 xc2x0.
A drawback of the known strip guiding device is that the service life of the supporting rolls is limited by the insulating action. The temperature of the supporting roll which is in operation and over which the hot metal strip is guided rises considerably, which has an adverse effect on the service life of the supporting roll. In addition, the high temperature increases so-called xe2x80x9cpick-upxe2x80x9d phenomena, i.e. the accumulation of small particles on the running surface of the supporting roll, leading to damage to the hot metal strip which is guided over it. In order to reduce the temperature of the supporting roll, it is possible, inter alia, to arrange cooling means inside the supporting rolls. In the case of so-called fiber rolls, i.e. supporting rolls which are coated with fiber discs, however, the cooling action is limited owing to the insulating action of the fiber material. In practice, it has been found that indirect cooling of an operating supporting roll which is accommodated in a rotatable support construction by means of internal cooling lines does not sufficiently reduce the temperature of the running service of this supporting roll. It is also known, in the case of metal supporting rolls with a metal outer casing, to guide cooling water directly along the inside of the outer metal casing. However, this results in very high heat losses from supporting rolls of this nature, and consequently the local cooling, particularly in the case of thin metal strip which is to be annealed, on the outer casing of the supporting roll is so high that it may lead to deformation of the metal strip.
A strip guiding device is also known from U.S. Pat. No. 4,039,372. This patent shows a rotatable support construction for two supporting rolls of a strip guiding device. Between the supporting rolls an isolation shield is provided having a central cooling chamber surrounded by top and bottom layers of insulation material. The shield provides insulation against the high temperatures in the furnace chamber for the supporting roll which is not in operation. Each supporting roll is provided with an internal cooling liquid pipe extending over the entire length of the supporting roll.
A drawback of this known strip guiding device is that the cooling means inside the supporting rolls are not able to provide for a uniform and sufficient cooling of the running surface of the supporting roll in operation. Like the device of DE-C-3401792, the service life of the supporting rolls used with the support construction according to U.S. Pat. No. 4,049,372 is limited, leading to damages to hot metal strip guided over it and limitations in production capacities and furnace temperatures.
The object of the invention is to overcome the above mentioned drawbacks, and, in particular, to provide a strip guiding device in which supporting rolls can be changed quickly, while at the same time a maximum possible service life of the supporting rolls at high furnace temperatures and high production capacities is also achieved. In particular, the object of the invention is to provide a strip guiding device in which good service lives can be achieved even for supporting rolls which are covered with fiber discs. Supporting rolls of this nature are preferably used for guiding this, cold-rolled metal strip, which is highly susceptible to damage.
According to the invention, these objects are achieved by means of a strip guiding device for guiding hot metal strip, comprising a frame, a support construction which is accommodated in said frame rotatable about an axis, drive means for stepwise rotation of said support construction in said frame, at least two supporting rolls which are rotatably and removably arranged in said support construction, insulating means which are connected to said support construction and are positioned between said supporting rolls, and cooling means for cooling each supporting roll. The cooling means for each supporting roll comprise a cooling body which is connected to said support construction and extends in a longitudinal direction of the associated supporting roll, wherein each cooling body externally delimits said associated supporting roll over more than 90xc2x0 of its circumference, and is positioned at a distance from said support roll for cooling said associated supporting roll as a result of direct radiation. The strip guiding device comprises a support construction which is mounted in a frame and can be rotated in steps by means of drive means. At least two rotatable and exchangeable supporting rolls are arranged in the rotatable support construction. The supporting rolls are thermally insulated from one another by means of insulating means which are connected to the support construction. An external cooling body is provided for each supporting roll. The cooling bodies are connected to the support construction, each cooling body externally surrounding the associated supporting roll over at least a quarter of its circumference. There is a small space between the cooling bodies and the supporting rolls. In operation, the top supporting roll, over which hot metal strip is guided, is cooled as a result of direct radiation to the cooling body. This allows the running surface of the supporting roll to be cooled in a very uniform but not excessively intensive manner. The resultant uniform, limited reduction in the temperature of the running surface significantly improves the service life of the supporting roll and considerably reduces pick-up, or even eliminates this phenomenon altogether. It has been found that with the strip guiding device according to the invention, fiber rolls can be used up to a few weeks even at very high temperatures, whereas in the devices used hitherto the supporting rolls often had to be replaced after only a few days. A further advantage is that a supporting roll which has been rotated out of its operating position can be cooled rapidly and uniformly, with the result that maintenance or replacement of the supporting roll can be carried out quickly. The efficient cooling of the running surface by means of direct radiation of heat from the roll advantageously allows a higher operating temperature of a furnace to be established without this leading to adverse xe2x80x9cpick-upxe2x80x9d phenomena or defects on the material of the supporting rolls. Furnace temperatures of up to approx. 1250xc2x0 C. are advantageously quite possible. In practice, it has been found that the temperature of the running surface of the operating supporting roll, via the cooling body which delimits more than 90xc2x0 of the circumference of the supporting roll, is cooled sufficiently to allow the supporting roll to rotate at high speeds, thus allowing high production capacities.
It should be noted that FR-A-1,370,251 has disclosed a strip guiding device with bearing rolls which are accommodated in cavities in a solid thermally insulating body. Beneath each cavity, there is a cooling-liquid pipe for cooling the bearing roll via direct radiation of heat. However, this construction as such is relatively unsuitable for use in a strip guiding device comprising a rotatable construction for changing supporting rolls. The removal of heat to the single, narrow cooling-liquid pipe in the bottom of the cavity is extremely limited, and consequently the circumferential speed of the supporting rolls has to be adapted in order to achieve a desired mean temperature for the supporting rolls. At furnace temperatures of over 1000xc2x0 C., this mean supporting-roll temperature can only be sufficiently low to prevent sagging of the supporting rolls. The xe2x80x9cpick-upxe2x80x9d phenomenon and other damage to the surface structure of the supporting rolls will still be present. In particular, fiber rolls cannot be used in the construction described in FR-A-1,370,251, since even a temporary standstill of the supporting rolls would within a very short time damage these rolls to such an extent that they would have to be replaced.
The advantage of the construction according to the invention is that it can be used for all types of supporting rolls. For example, it can be used to guide hot-rolled metal strip with an oxide skin over metal supporting rolls provided with a bronze coating. In particular, so-called fiber rolls, in which, for example, metal supporting rolls are covered with fiber discs, can be successfully cooled on their running surface by means of the cooling construction according to the invention. In this way, it is possible for even thin, cold-rolled metal strip, which is highly susceptible to damage, to be guided successfully. It is noted that effective cooling of the running surface of fiber rolls with the heat being dissipated to a cooling body located inside the fiber roll is impossible, or only possible with great difficulty, owing to the very low coefficient of thermal conductivity of the fiber discs.
In particular, the cooling body delimits more than 140xc2x0 of the circumference of the associated supporting roll. The extensive surrounding delimitation of the supporting roll by the cooling body ensures that there is sufficient cooling of the supporting roll even if the metal strip, and consequently the supporting roll, are temporarily at a standstill. This is advantageous since, in the known strip guiding devices, if the hot metal strip is at a standstill the support construction has to be rotated through an angle of 90xc2x0, in order to prevent damage to the supporting roll as a result of overheating. In the case of the strip guiding device according to the invention, rotation of this nature is only necessary in the event of a defect to the cooling body or the cooling system.